Relatives of India’s first president have sought the intervention of the country’s Ambassador to Switzerland to try and halt the sale of a rare watch once owned and worn by the man who played a seminal role in the nation’s history.

The great grandson of the late Rajendra Prasad has written to the embassy in Geneva, requesting it to step in and try and ensure the 18 carat pink gold Rolex Oyster is somehow returned to India. The auctioneers insist the watch is being sold legitimately and estimate it will fetch up to £140,000 when it comes under the hammer next month.

In an emailed message to the Indian ambassador in Switzerland, Ashoka Jahnavi Prasad, the eldest great grandson of the former president, said his family had only come to learn the watch had emerged decades after they last saw it, and were anguished by news of its imminent sale.

“We have no idea how the watch reached Geneva but can confirm that the family had nothing to do with it,” he wrote. “We also believe the item rightly belongs to the nation, to which we shall remain forever indebted for enabling our forebear to serve it…We should all feel most grateful if you could kindly look into the matter.”

The late Dr Prasad is a revered figure in India, celebrated for both his role in helping secure the country’s independence and well as drafting its first constitution. He served as the country’s first president and it is believed he was given the watch to mark India’s first republic day celebration on 26 January 1950. (India had secured independence in 1947, but its constitution was not completed for a further three years.)

The dial of the watch is said to be decorated with a map of India and the date of that republic day event. Nepal and Afghanistan are also shown on the watch face.

Mr Prasad, 56, told The Independent, that his great grandfather, a man of humble tastes, wore the watch only on special occasions, and that it was one of two timepieces specially made to mark the occasion when India’s constitution came into force. The other, he said, was given to the country’s first prime minister, Jawaharlal Nehru.

Mr Prasad, a medical consultant, said that following his great grandfather’s death in 1963, many of his possessions, including the watch, were donated to a museum in Patna, capital of the state of Bihar where he spent his childhood and retirement. He said that sometimes in the mid-1960s, the watch “mysteriously disappeared”. He added: “We accepted the fact that we would never see it again. Then we read about this sale in the newspaper.”

A diplomat with the Indian embassy in Geneva said they were aware of Mr Prasad’s email and that an official would look into the matter. “I am sure the appropriate official will be taking action,” said the diplomat.

Officials with the Bihar branch of the Congress Party, of which the late Dr Prasad was a member, told the Times of India they were taking up the issue as a matter of urgency. “This is a serious issue and cannot be overlooked,” said SK Verma, a spokesman.

The watch is due to be put on sale on 13 November by Sotheby’s as part of a larger lot of timepieces, including one owned by Konrad Adenauer, the first chancellor of what was West Germany. A senior spokesman, Matthew Weigman, said the seller of the Indian watch wished to remain anonymous. He said there had been no contact from either the Indian embassy or a member of Dr Prasad’s family. He added: “Sotheby’s had followed all of its customary due diligence procedures before accepting the watch for sale, including checking the consignor’s title. Sotheby’s established that the watch had never been reported as stolen to the Art Loss Register, the database of missing or stolen art.”